Study finds Mt. Fuji erupted 43 times in 2,000-year period

TOKYO--Mt. Fuji has erupted and discharged lava at least 43 times in the past 2,000 years, according to a study by Japan's National Institute of Advanced Industrial Science and Technology (AIST).

The findings are a result of geological surveys over a 15-year period aimed at revising Mt. Fuji's geological map, which currently has many gaps.

The AIST, based in Tsukuba, Ibaraki Prefecture, unveiled a draft of an updated geological map at an international conference in Kagoshima last week. The draft reflected newly found indications of past eruptions.

The survey on rocks and areas at the foot of Mt. Fuji faced greater delays than studies of other major volcanoes because Mt. Fuji's base is much wider. On the current map, published in 1968, most areas on the mountain's eastern slope are blank.

A team led by senior AIST research scientist Takahiro Yamamoto has surveyed about 900 square kilometers of the iconic mountain since 1998.

Taking advantage of the characteristics of lava, whose radiocarbon changes with time, the team examined the composition and variety of rocks in the areas and used radiometric dating of volcanic layers to determine when Mt. Fuji had erupted.

Mt. Fuji has been dormant for more than 300 years. However, the team found that lava was discharged 16 times between about 50 B.C. and A.D. 1100 based on new investigations on the eastern slope alone. The survey concluded that Mt. Fuji has erupted 43 times in total over the past 2,000 years.

During a large-scale eruption on the eastern slope around A.D. 50, lava reached as far as about 700 meters above sea level, which is near Gotemba, Shizuoka Prefecture, where many lodging facilities are located today. The new geological map, revised for the first time in 45 years, is scheduled to be completed this fiscal year and is expected to be incorporated into municipal evacuation plans.

Toshitsugu Fujii, an honorary professor at the University of Tokyo and head of the Coordinating Committee for the Prediction of Volcanic Eruptions, said: “A geological map of a volcano can serve as the basis for hazard maps that show evacuation routes and other information. It's revolutionary that a detailed geological map covering such a wide area of Mt. Fuji will become available.